ABSTRACT 1: In electron micrographs the rhabdomeric microvilli were longer and narrower, the area of contiguous microvillar membrane greater, the endoplasmic reticulum less abundant, and the mitochondrial granules (? calcium containing) more numerous in well dark adapted than in well light adapted retinular cells and subrhabdomeric phagocytotic vesicles were present in partially light adapted cells. On going and rhythmic spike activity and hyperpolarizing slow potentials have been recorded intracellularly from optic ganglion cells of Limulus. The role of retinular cell axons in modifying such activity will be determined. ABSTRACT 2: Chlorobutanol (0.05 to 1 mM; 1,1,1-trichloro-2-methyl-2 propanol) shortens the latency of the receptor potential of Limulus lateral eye retinular cells, increases membrane resistance to hyperpolarizing current, and higher concentrations depolarize the cells. The effect is temperature independent and the rate of change of current in cells clamped at resting potential is unchanged by chlorobutanol. Voltage clamp experiments of Limulus lateral eye retinular cells have revealed two components in the transient light-induced current at a variety of membrane voltages. The slope resistance of the C1 current is appreciably higher than that of the C2 current. Other indices indicate that membrane properties change abruptly but reversibly between negative 5 and plus 5 mV membrane potential.